1. Field of the Invention
This invention relates to a control system for an automotive vehicle equipped with an automatic transmission.
2. Related Art
It has been conventionally utilized in an automatic transmission for an automotive vehicle that a lock-up clutch is provided in a torque converter to mechanically connect the input and output sides of the torque converter to each other, and the lock-up clutch is jointed under a prescribed driving condition to prevent reduction in power transmission efficiency and thus aim at improving fuel consumption.
In addition, in order to further improve the fuel consumption, it has been considered that the lock-up clutch is also jointed at its coasting drive time at which an acceleration pedal is fully closed to gradually reduce engine rotation, thereby lengthening a so-called fuel cut period for which fuel supply to an internal combustion engine is cut when a throttle valve is fully closed and an engine rotating number is above a predetermined value.
However, if the lock-up clutch is set to the joint state (lock-up state) when a vehicle coasts as described above, the rotational speed of driving wheels would be quickly reduced when a driver jams the brakes on, so that engine stall is induced.
In order to solve the above problem, Japanese Laid-open Patent Application No. 63-4061 or Japanese Laid-open Patent Application No. 4-370465 proposes a device in which the deceleration speed of a vehicle is detected on the basis of brake fluid pressure or the like and the lock-up (joint of the lock-up clutch) of the torque converter is released when the deceleration value exceeds a predetermined value.
However, the above conventional device has the following problems.
Ordinarily, the lock-up clutch is controlled with hydraulic pressure which is obtained by actuating a solenoid valve, and there is a time lag from the time when the activation of the solenoid valve is started until the time when the joint of the lock-up clutch is actually released. Accordingly, when a driver quickly jams the brakes on, the engine stall finally occurs due to a response delay until the lock-up clutch is released.
For general automotive vehicles, a braking power is set to be stronger for front-wheel brakes than for rear-wheel brakes, and thus the driving wheels are braked by the stronger front-wheel brakes in a so-called FF vehicle (front engine and front drive vehicle). Therefore, the engine stall is more liable to occur in the FF vehicle. On the other hand, even in a so-called FR vehicle (front engine and rear drive vehicle), the release of the lock-up clutch is delayed on a road having low friction coefficient on which the wheels of a vehicle are liable to be locked at its braking time, so that the engine stall finally occurs.